During the Waterloo campaign, there were TWO Allied armies facing Napoleon, one under England's Wellington, and one under Prussia's Blucher. Napoleon first defeated Blucher at Ligny, then turned on Wellington's then outnumbered army at Waterloo. The latter was a close battle until the badly wounded 72-year-old Blucher arrived with the remnants of his badly defeated army to reinforce Wellington, late in the day. Then the combined forces crushed the French.

Could the Allies have avoided the defeat at Ligny, and the near-defeat at Waterloo by combining/uniting their forces earlier?

2 Answers
2

Yes. However, I don't think you're are giving Napoleon enough credit here as the driver of events. It appears that the entire point of Ligny was to prevent exactly that. Here's what wikipedia (currently) has to say:

The battle of Ligny is a prime example of a tactical win and a
strategic loss. However, had the left wing of Napoleon´s army
succeeded in keeping the Prussian army from joining the British Army
under Wellington at Waterloo, as the Emperor had planned, Napoleon
might have won the Waterloo Campaign.

In particular, it appears that Wellington thought that Napoleon's main aim was going to be to march around him and cut his communications, rather than to separate the allies and defeat them in detail. Napoleon made sure to feed Wellington false intelligence to reinforce this belief.

Some credit goes to Napoleon, certainly. The same, or another Wikipedia article also faults Wellington for not hastening inland to the support of Blucher, because he wanted to "cover" Brussels and the channel ports. And Blucher couldn't get to the coast fast enough.
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Tom AuJul 3 '12 at 22:09

@TomAu - Right. This was perhaps at least in part thanks to Napoelon's false intelligence though, so we are back to Napoleon. IMHO an enemy commander would have to be nearly as ham-handed as Bull Halsey at Leyte before I'd take the step of blaming him for being taken in by enemy deception. Lord knows I probably would do worse.
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T.E.D.♦Jul 3 '12 at 23:45

I happen to be an admirer of U.S. Grant, so my primary war objectives would be 1) fighting the enemy army and 2) uniting with allied armies.history.stackexchange.com/questions/805/…. Prior to Vicksburg, Grant "let go" of his communications (with the Mississippi) and marched inland on five days' rations to head off the relieving force. I would probably have done the same to join Blucher. (But would have been blamed for losing Brussels and the channel ports.)
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Tom AuJul 6 '12 at 13:24

@TomAu - I have to admit, I have trouble imagining Grant getting scared about his supplies like that. But then again, knowing how your opponent thinks and taking that into account is an important part of being successful too. If facing someone like Grant, perhaps Napoleon would not have used the same strategy. There's no way to know.
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T.E.D.♦Jul 6 '12 at 13:47

@ T.E.D. Grant was an "original," and therefore hard for the Confederates to defeat. Even after their tactical victory at the Wilderness, Grant ordered his "retreating" army SOUTH> And Blucher was another one of this kind. Not many badly wounded 72-year-old generals lead their defeated armies on a "revenge march" to reinforce their allies.
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Tom AuJul 19 '12 at 0:41

It was unknown amongst the Allied armies in which direction Napoleon would march.
He might have chosen to march into Belgium, which in addition to cutting Wellington off from supply could have led the Belgians to rise in revolt against the Kingdom of the Netherlands, of which they had only lately been forced to become a part.
He might have marched on Prussia instead - and I may be wrong, but I believe the Prussian Army had not yet fully concentrated, and so Blucher covered the route between Napoleon and Prussia.
In any case, the Allies expected to have to concentrate to fight Napoleon, and their plan was to do so once they knew the axis of his advance.
In order to do so, they would have had to have better scouting - and Wellington would have needed greater confidence that Napoleon was not hooking past him at Mons, or at least earlier scouting reports.
This is where the battles of Ligny and Quatre Bras come in - the French attacked the Prussians at Ligny, sending them reeling back in retreat, and at Quatre-Bras prevented Wellington's troops from advancing in time to assist the Prussians at Ligny.
Having thus cleared the Prussians from his right (as he thought) Napoleon turned on Wellington at Waterloo. Wellington and Blucher (or possibly Blucher's staff, given the injuries he took at Ligny) were in communication throughout the 17th, and so Wellington stood at Waterloo well aware of the fact that Blucher was coming. He had no guarantee that Blucher would arrive in time, but if the Prussians had been forced into a general retreat rather than re-grouping at Wavre, Wellington would have continued retreating.
In other words, the course of events clearly shows that Wellington and Blucher intended to force Napoleon to fight them together, and that Napoleon attempted, with incomplete success, to drive them apart so that he could defeat them in detail. So yes, an earlier link-up between the Allied armies would have meant Napoleon's efforts failed earlier.